Practical applications of portable electronic appliances such as video cameras, cellular phones, portable PCs, etc., has brought about an increasing interest in secondary batteries generally used to provide operating power thereto. In particular, lithium secondary batteries are increasingly used, since they have a high energy density per unit weight and can be rapidly charged, as compared to other conventional batteries such as lead acid batteries, nickel-cadmium (Ni—Cd) batteries, nickel-hydrogen (Ni—H2) batteries and nickel-zinc (Ni—Zn) batteries.
Unlike conventional non-rechargeable primary batteries, a great deal of research is conducted on secondary batteries enabling charge/discharge associated with development in high-tech industries such as digital cameras, cellular phones, notebook computers, and hybrid automobiles. Such secondary batteries may be classified into nickel-cadmium batteries, nickel-metal hydride batteries, nickel-hydrogen batteries, lithium secondary batteries, and the like. Of these, lithium secondary batteries, which are used as power sources for portable electronic appliances or are connected in series for use in high-output hybrid automobiles, have a driving voltage of 3.6V or higher. Use of lithium secondary batteries is rapidly increasing, due to 3-fold higher driving voltage and superior energy density per unit weight, as compared to nickel-cadmium batteries or nickel-metal hydride batteries.
Depending on the type of electrolyte used, lithium secondary batteries may be classified into lithium ion batteries utilizing liquid electrolytes and lithium ion polymer batteries utilizing solid polymer electrolytes. Depending on the type of solid polymer electrolyte used, lithium ion polymer batteries may be classified into perfect solid-type lithium ion polymer batteries containing no electrolyte solution and lithium ion polymer batteries using gel-type polymer electrolytes comprising an electrolyte solution.
Lithium ion batteries using liquid electrolytes are generally welding-sealed in the form of a cylindrical or prismatic metal can. Can-type secondary batteries using such a metal can as a container have a constant shape, thus disadvantageously having limited design of electronic appliances using the batteries as power sources and difficulty in reducing the volume. Accordingly, pouch-type secondary batteries wherein two electrodes and a separation membrane and an electrolyte are placed in a film-type pouch and the pouch is sealed, have been developed and put to practical use.
FIG. 1 illustrates a pouch-type secondary battery. The pouch includes a lower sheet 20 provided with an accepting portion 21 and an upper sheet 10 covering the same. An electrode assembly 30 housed in the accepting portion 21 is formed by rolling a laminate of a cathode 31, an anode 35 and a separator 33. The electrode assembly is accepted in the accepting portion 21, an upper sheet 10 and a lower sheet 20 are thermally-bonded to form the sealing portion 23, electrode taps 37 and 38 protrude from the respective electrodes, and a tape 39 may be adhered in a region where the electrode taps 37 and 38 overlap the sealing portion 23.
The pouch including the upper sheet 10 and the lower sheet 20 will be described based on the upper sheet 10. The pouch has a multi-layer structure including an inner layer 15 (a polyolefin layer) acting as a sealant due to thermal-bonding properties thereof, a metal layer 13 (aluminum layer) made of a material maintaining mechanical strength and acting as a barrier layer against moisture and oxygen, and an outer layer 11 (generally a nylon layer) which are laminated in this order. Casted polypropylene (CPP) is commonly used as a polyolefin-based resin layer.
In the process of fabricating a battery with such a pouch including the upper sheet and the lower sheet, the electrode assembly 30 including an electrode and a separator is rolled and thermally-bonded to form the sealing portion 23 in which the upper sheet is adhered to the lower sheet, as shown in FIG. 2. However, the thermal-bonding portion of the pouch is vulnerable to permeation of external moisture. Accordingly, long-term permeation of moisture is inevitable and the permeated moisture thus reacts with LiPF6− contained in the electrolyte solution to produce HF. As a result, an anode active material is deteriorated.
Disadvantageously, the pouch-type secondary batteries may have a variety of shapes and realize secondary batteries exhibiting a predetermined capacitance with a lower volume and weight. However, unlike can-type batteries, pouch-type batteries use a soft pouch as a container and thus exhibit low mechanical strength and reliability. Accordingly, pouch-type batteries are generally applied to gel- or perfect solid-type lithium ion polymer batteries rather than lithium ion secondary batteries using an electrolyte solution having the serious problem of solution leakage.
However, pouch-type batteries should include electrodes and an electrolyte such that they can exhibit higher capacitances in response to the demand for high-capacitance secondary batteries. In addition, there is a demand to gradually decrease the size of the sealing portion which is not directly related to capacitance or capacity of batteries.
The reason for such demand is that an electrode assembly with a higher capacitance can be accepted in the pouch and the sealing portion unrelated to the capacitance can be decreased in size, as the width of the sealing portion of the pouch decreases. Accordingly, as the width of the sealing portion decreases, secondary batteries with high capacitance, as compared to a pouch of identical size, can be formed.
However, an absolute sealing area is decreased and sealing reliability of the pouch is deteriorated due to the decreased width of the sealing portion. External moisture is permeated into the pouch, which disadvantageously causes deterioration in long-term storage stability and thus battery performance. There is a need for a pouch capable of solving these problems.